1. Field of the Invention
The present invention relates to improvements in a hydraulic control system for a continuously variable transmission of a belt-and-pulley type for an automotive vehicle.
2. Discussion of the Related Art
A belt-and-pulley type of continuously variable transmission for an automotive vehicle is known. This transmission includes a first shaft, a second shaft, a first variable-diameter pulley provided on the first shaft, a second variable-diameter pulley provided on the second shaft, a transmission belt connecting the first and second pulleys to transmit power from the first shaft to the second shaft, a first hydraulic cylinder for changing an effective diameter of the first pulley, and a second hydraulic cylinder for changing an effective diameter of the second pulley. An example of a hydraulic control system for this type of continuously variable transmission is disclosed in Laid-open publication No. 52-98861 (published in 1977) of Japanese Patent Application, wherein the belt tension is adjusted primarily by regulating the pressure of a flow of a working fluid supplied to the second hydraulic cylinder (provided on the second or driven shaft), whereas the speed ratio of the transmission is controlled primarily by regulating the pressure of a flow of the fluid supplied to or discharged from the first hydraulic cylinder (provided on the first or drive shaft).
In such a hydraulic control system, a single line pressure regulated in relation to the speed ratio of the transmission is provided to activate the second hydraulic cylinder primarily for maintaining a suitable tension of the transmission belt. The line pressure is also applied to the first hydraulic cylinder, via a flow control valve which functions to control the speed ratio of the transmission. In this arrangement, the rates of flow of the fluid to and from the first cylinder change with the line pressure, that is, with the speed ratio of the transmission. Therefore, it is inevitable that the rate at which the speed ratio is changed is affected by the specific current speed ratio. Accordingly, the above-indicted hydraulic control system tends to fail to provide a sufficient response in the control of the speed ratio, while the transmisson is in a transient condition, that is, operating at a changing speed ratio. Further, when the direction of power transmission is reversed due to an engine braking effect, the belt tension is primarily controlled by the first cylinder while the speed ratio is regulated by the second cylinder, whereby the belt tension and the speed ratio cannot be suitably controlled.
Another hydraulic control system for a continuously variable transmission is disclosed in Publication No. 58-29424 (published in 1983 for opposition purpose) of Japanese Patent Application, wherein a control valve (four-way valve) is used to change the speed ratio of the transmission by feeding a working fluid from a hydraulic source to one of the two hydraulic cylinders, and at the same time allowing the fluid to be discharged from the other cylinder. The control system further uses an electromagnetic relief valve for regulating the pressure of the fluid which is delivered from the control valve.
In the hydraulic control system indicated above, a comparatively high pressure is applied to the hydraulic cylinder for the pulley provided on the drive shaft, i.e., to one of the two cylinders which is subject to a relatively high pressure due to power transmission in the direction from the drive shaft to the driven shaft of the transmission. The other cylinder is supplied with the pressure which is regulated by the electromagnetic relief valve. According to this arrangement, the belt tension and the speed ratio of the transmission can be suitably controlled, even when the direction of power transmission is reversed.
Problems Solved by the Invention
In the conventional continuously variable transmission, however, the pressure of the fluid delivered by the hydraulic source is not regulated, but is merely maintained at a predetermined level by an ordinary pressure relief valve. Hence, the rate of change in the speed ratio or the shifting response of the transmission may have relatively large variations, due to changes in the fluid pressure levels within the hydraulic cylinders according to a change in the torque transmitted by the transmission and a change in the speed ratio of the transmission. It is possible to set the pressure of the hydraulic source at a level high enough to permit a sufficiently high rate of change in the speed ratio over the entire operating range of the transmission. In this case, however, the system suffers from increased power loss due to maintenance of the high pressure throughout the operation of the transmission.